Since a surface-mount type quartz crystal unit having a quartz crystal blank hermetically encapsulated in a container for surface mounting is small and light, it is incorporated, as a frequency control element, in an oscillation circuit of a variety of portable electronic devices, for example. In the surface-mount type crystal unit, a metal cover is generally used so as to hermetically seal the container. It is desired to electrically ground the metal cover to suppress EMI (electromagnetic interference). In recent years, miniaturization of the surface-mount type crystal unit progresses, and, for example, a planar projection size thereof becomes 2.0 mm×1.6 mm or less.
FIG. 1A is a sectional view showing an example of a conventional surface-mount type crystal unit, FIG. 1B is a plan view showing of a state of the conventional crystal unit in which a metal cover is removed, FIG. 1C is a bottom view showing an outer bottom surface of the conventional crystal unit, FIG. 2A is a plan view showing an opening end face (i.e., upper surface) of a frame wall layer in the conventional crystal unit, FIG. 2B is a bottom view showing a stack surface (i.e., lower surface) of the frame wall layer with a bottom wall layer in the conventional crystal unit, and FIG. 2C is a plan view showing a stack surface of the bottom wall layer with the frame wall layer in the conventional crystal unit.
The shown surface-mount type crystal unit uses container body 1 having a recess at one principal surface and having an approximately rectangular parallelepiped shape. Crystal blank 2 is received in the recess that is then covered by metal cover 3. Thereby, crystal blank 2 is hermetically sealed in container body 1. Container body 1 is made of laminated ceramics comprising: bottom wall layer 1a having an approximately rectangular shape; and frame wall layer 1b stacked on bottom wall layer 1a and having a rectangular opening at a center thereof. Container body 1 has the recess that is formed by the opening of frame wall layer 1b. Hence, an inner bottom surface of the recess of container body 1 is an exposed portion of the upper surface of bottom wall layer 1a. The inner bottom surface of the recess is provided with a pair of crystal holding terminals 4 that are near both ends of one short side of the inner bottom surface, respectively. In other words, crystal holding terminals 4 are arranged on one end portion side of container body 1. On the outer bottom surface of container body 1, i.e., the bottom surface of frame wall layer 1a, a pair of diagonal corners, i.e., both ends of one diagonal line are provided with external terminals 5a, 5d for crystal blank 2 and the other corners are provided with grounding external terminals 5c, 5b. These external terminals 5a to 5d are used to surface-mount the crystal unit on a wiring board.
External terminals 5a, 5d are electrically connected to crystal holding terminals 4 via end face metal films 6c formed on an outer side surface of container body 1 and electrically conducting path 6a provided on the stack surface between bottom wall layer 1a and frame wall layer 1b. One end of electrically conducting path 6a is integrally formed with crystal holding terminal 4 on the upper surface of bottom wall layer 1a, as shown in FIG. 2C. The other end of electrically conducting path 6a is extended to the position of the outer side surface of container body 1 and connected to end face metal film 6c. 
The opening end surface of container body 1, i.e., an entire surface of a surface surrounding the recess, which is the upper surface of frame wall layer 1b, is formed with sealing metal film 7a, as shown in FIGS. 1B and 2A. In addition, as shown in FIG. 2B, on a lower surface of frame wall layer 1b, i.e., stack surface with bottom wall layer 1a, first conduction metal films 7b are provided near centers of the respective short sides of frame wall layer 1b. First conduction metal films 7b are connected to sealing metal film 7a through end face metal films 7c formed on inner side surfaces of frame wall layer 1b. As shown in FIG. 2C, on an upper surface of bottom wall layer 1a, i.e., stack surface with frame wall layer 1b, second conduction metal films 6b are provided such that films 6b are extended from the centers of the respective short sides to the corners corresponding to positions of external terminals 5c, 5b along the relevant short sides. Second conduction metal films 6b are electrically connected to grounding external terminals 5c, 5b through end face metal films 6c provided on the outer side surfaces of container body 1. By stacking bottom wall layer 1a and frame wall layer 1b, first and second conduction metal films 7b, 6b are electrically connected to each other at the centers of the respective short sides. Thereby, sealing metal film 7a is electrically connected to grounding external terminals 5c, 5b. 
When manufacturing the container body by stacking and firing ceramic sheets, first and second conduction metal films 7b, 6b are integrally formed, together with crystal holding terminals 4 and electrically conducting path 6a, with container body 1 by forming tungsten (W) films with printing method and the like for the ceramic sheets corresponding to frame wall layer 1b and bottom wall layer 1a before the stacking and firing, and then stacking and firing the ceramic sheets.
Crystal blank 2 consists of, for example, an AT-cut quartz crystal blank having an approximately rectangular shape. The both principal surfaces of crystal blank 2 are formed with excitation electrodes 8, respectively. From the pair of excitation electrodes 8, extraction electrodes 9 are extended toward both sides of one end of crystal blank 2. Extraction electrodes 9 at the ends of crystal blank 2 are folded between both principal surfaces of crystal blank 2. By adhering extraction electrodes 9 to crystal holding terminals 4 with conductive adhesive 10 and the like at positions at which the pair of extraction electrodes 9 are extracted, crystal blank 2 is fixed and held in the recess of container body 1 and electrically connected to external terminals 5a, 5d. 
Metal cover 3 for encapsulating crystal blank 2 in container body 1 is provided with a layer of eutectic alloy 11 along at least an entire circumference of a peripheral portion of one principal surface of the metal cover. The layer of eutectic alloy 11 is formed by, for example, melting and applying the alloy to metal cover 3. Eutectic alloy is gold-tin (Au—Sn) alloy, for example. By contacting the peripheral portion of one principal surface of the metal cover to the upper surface of the opening end surface of container body 1, i.e., the upper surface of frame wall layer 1b, and melting eutectic alloy 11 again, metal cover 3 is bonded to container body 1 and the opening end surface is sealed to hermetically close the recess.
In such a crystal unit, when eutectic alloy 11 is heated to melt upon sealing metal cover 3, molten eutectic alloy 11 is concentrated on the four corners having a relatively large area. Hence, when the connection positions of first and second conduction metal films 7b, 6b are the corners of frame wall layer 1b, there are provided the end face metal films that are used for connection with sealing metal film 7a at the positions of the corners. However, in this case, there is some possibility that the molten eutectic alloy is transferred through the end face metal films and flowing down along an inner wall of the recess, and then reaches crystal holding terminals 4. When the eutectic alloy flows to crystal holding terminals 4, crystal holding terminals 4 are electrically shorted to a ground potential and thus the crystal unit does not function. In addition, when a large amount of the eutectic alloy is introduced into the recess and the eutectic alloy is thus attached to crystal blank 2, a vibration characteristic of crystal blank 2 is deteriorated.
On the other hand, as described above, in the structure that first conduction metal film 7b is provided to the center of each short side of frame wall layer 1b and electrically connected to sealing metal film 7a by end face metal film 7c at the center of each short side, eutectic alloy 11 does not electrically short with crystal holding terminals 4 and eutectic alloy 11 is suppressed from being introduced into the recess. As a result, it is possible to prevent the eutectic alloy from being attached to crystal blank 2.
However, in the surface-mount type crystal unit shown in FIGS. 1A to 1C and 2A to 2C, on the stack surface between bottom wall layer 1a and frame wall layer 1b, second conduction metal films 6b composed of tungsten, for example, are formed toward the corners from the centers of the respective short sides, so that the strength of container body 1 against mechanical impact is decreased. In other words, since the bonding strength between a metal film and ceramic is lower than that between ceramics, the bonding strength between bottom wall layer 1a and frame wall layer 1b is decreased due to the provision of second conduction metal films 6b on the stack surface between bottom wall layer 1a and frame wall layer 1b, compared to a case where metal films 6b are not present. According to the experiment performed by the inventors, it was proven that strength against a horizontal impact to container body 1 is decreased, rather than a vertical impact to the container body. When the horizontal impact is applied, there is generated delamination at a boundary surface between bottom wall layer 1a and frame wall layer 1b. 
A crystal unit capable of grounding a metal cover without a conduction metal film formed on a stack surface between a bottom wall layer and a frame wall layer is disclosed in a Japanese Patent Laid-Open Application No. 2004-146956 (JP-2004-146956A). In this crystal unit, via-holes are provided to the frame wall layer and the bottom wall layer, thereby electrically connecting the metal cover to a grounding external terminal. However, according to this structure, since the via-hole is formed, it is required to extend a frame width of the frame wall layer as much as the hole, so that a miniaturization of the crystal unit is hindered. In addition, the manufacturing processes are increased, so that manufacturing costs are increased.